JavaScript实现遮罩层与弹出注册界面

<head>
 <meta charset="UTF-8">
 <title>遮罩</title>
 <style>
  * {
   margin: 0;
   padding: 0;
  }
  #shade{
   position: fixed;
   width: 100%;
   height: 100%;
   display: none;
  }
  .shade-background{
   width: 100%;
   height: 100%;
   background-color: #CCCCCC;
   opacity: 0.5;
  }
  #register{
   position: relative;
   background-color: #fff;
   width: 500px;
   height: 300px;
   top: -70%;
   margin: 0 auto;
  }
 </style>
</head>
<body>
<div id="shade">
 <div class="shade-background"></div>
 <div id="register">
  用户名<input type="" name="" id="" value="" /><br />
  密&nbsp;码<input type="" name="" id="" value="" /><br />
  <input type="submit" name="" id="reg" value="注册" />
 </div>
</div>
<div>
 <input type="button" name="" id="btn" value="弹出遮罩" />
</div>
</body>
<script>
 var shade = document.getElementById("shade");
 var btn = document.getElementById("btn");
 var reg = document.getElementById("reg");
 btn.onclick = function(){
  shade.style.display = "block"
 }
 reg.onclick = function(){
  shade.style.display = "none"
 }
</script>